Executive Summary

Pinniped predation on fish farms is a worldwide problem and
causes the industry financial losses of up to 10% of the total farm
gate value (Nash et al. 2000). This has led to a need for the
industry to look at non-lethal measures controlling seal damage,
which include tensioning nets, deployment of a predator net or use
of Acoustic Deterrent Devices (
ADDs)
(Würsig and Gailey 2002).
ADDs have
often been considered a non-harmful method of dealing with the
problem. However, the main problems with
ADDs appear to
be a lack of long-term efficiency and unintended effects on other
marine wildlife (Jefferson and Curry 1996). A recent study has
found a new method of acoustic deterrence using the acoustic
startle reflex (Gotz & Janik 2011), which proved successful in
deterring seals and avoiding effects on harbour porpoises over a
two month period. This project tested (a) the effects of startling
sounds on seal predation and marine mammal abundance around a test
farm compared to adjacent control farms without
ADDs over a 13
months period and (b) determined the startle threshold for
bottlenose dolphins to prepare the method for use in other
applications such as marine construction. The project also tested
the short-term effectiveness of the startle method on two
additional farms when they experienced high seal predation
rates.

The use of the startle method resulted in a highly significant
reduction in the number of lost fish on the long-term test site
compared to the pre-deployment period (Mann-Whitney U, n=16, U=38,
p=0.004, Fig 3). In fact, median losses per month were zero on the
test site when the sound was played. This was a highly significant
difference in predation losses compared to the control sites (Poll
na Gile, Mann-Whitney U, n=22, U=103, p=0.001; Ardmaddy, n=21,
U=20.5, p=0.01). Median losses were 41 fish/month on control site 1
(Poll na Gile), 39 fish/month on control site 2 (Ardmaddy), 98
fish/month on the test site before the deployment of the startle
equipment and zero fish per month when the equipment was operating
(Fig 3). There were only 5 consecutive events of negligible to
moderate predation on the test site during the 13 months study,
which consisted of 58, 14, 7, 5 and 1 fish losses. The direct
comparison of monthly losses between the pre-deployment period,
test period and control sites showed that the startle method was
capable of reducing predation losses significantly throughout the
one year deployment (Fig 3). This was also confirmed by a
statistical model which showed that sound exposure was the most
important explanatory factor with respect to variation in seal
predation. The model also revealed that predation varied throughout
the year, although different sites did not differ in their losses
during different times of year. Similarly, overall predation levels
did not differ across sites. Seals, porpoises and otters approached
the farm throughout the entire test period. There was no
significant difference in number of seals and porpoises at
different distances from the fish farm throughout the test
period.

Rapid response trials at two fish farms with high seal predation
rates also proved highly successful. At the first farm 405 fish
were killed by seals in the month before the startle equipment was
deployed (Fig 8). Dive reports after deployment of the equipment
showed no new, seal-related kills for 2 weeks at which time the
farm was harvested. At the second farm predation also dropped to
zero in the first week after deployment. However, the equipment was
damaged in a storm afterwards and predation levels returned to the
high levels found prior to deployment.

Our tests showed that the startle method was highly successful
in limiting seal predation at an operating fish farm with no
evidence for habituation during a 13 month period. Similarly, the
method was highly successful at limiting predation in cases where
predation pressure was high. A likely explanation for the five
events of predation while the startle equipment was operating in
the long term test is that the predating individuals could have had
compromised hearing which could be either the result of genetic
predisposition, disease, old age or previous exposure to
anthropogenic noise source (such as commercially available seal
scarers).

Movement data of marine mammals showed that the startle method
did not influence the distribution of harbour seals, porpoises and
otters. The fact that harbour seals still approached the farm quite
closely when the startle equipment was operating is in contrast
with previous findings when measuring approaches. However, in the
previous test sound exposure was more varied and lasted for only 2
months. It is therefore likely that seals observed at the surface
near the fish farm did try to avoid the sound by swimming with
their heads above the water. The fact that there was virtually no
predation confirms that the sound had an effect on seals. Harbour
porpoises were also observed at the surface near the equipment, but
kept their heads underwater confirming that the sound did not have
an effect on them. The deterrence system tested in this study
operated at a duty cycle of less than 1% which is between one and
two orders of magnitude lower than in current commercially
available deterrent devices. The fact that brief, isolated pulses
were emitted at only moderate levels means that noise pollution was
greatly reduced and the potential for masking of communication
signals or hearing damage is low. This is in contrast to current
commercially available
ADDs which
emit sound at high duty cycles and high source levels. We would
recommend the use of this novel technology at fish farms.

The startle method would potentially also be useful to
temporarily deter cetaceans from marine construction sites. One
possible problem with the application for echolocating toothed
whales is that these animals produce very loud echolocation pulses
and therefore might have an auditory mechanism to avoid startling
themselves. We therefore tested whether bottlenose dolphins would
startle to pulsed sounds and what their startle threshold would be.
We used two captive bottlenose dolphins to conduct tests of their
reactions when listening to startle sounds. The startle was
quantified through an accelerometer attached to the animal that
recorded any kind of muscle flinches during playbacks of sounds. We
found that both animals clearly startled to our pulses and that the
startle threshold for this species lies at around 80 dB above their
hearing thresholds. Since we have shown here that echolocating
animals also startle, the method is likely to work also with
harbour porpoises. Our results allow us now to design startle
sounds specifically for dolphins and porpoises. However, further
tests to see whether dolphins and porpoises sensitize in the same
way as seals are still needed. If they do, the startle method can
be used to deter either only seals, only dolphins and porpoises, or
all of these taxa.